Government regulations have driven the replacement of coatings having volatile organic content (VOC) with water-borne coatings. However, water-borne coatings are much slower to dry, and in some cases they must be applied sparingly to achieve reasonable dry rates and to avoid excessive shrinkage. Conventional water-borne coatings typically require the application of several layers of coating and results in high labor cost and high overall cost of their use.
Generally, the higher the solids content by volume of liquid coating, the faster it dries resulting in a thicker, dried coating. The solids content of a liquid coating as a measure of volume is commonly referred to as the volume-non-volatile (NV/volume). To increase the drying rate as well as the coverage of a water-borne coating, it is a common practice to increase the proportion of non-film-forming solids by adding fillers such as kaolin clays or calcium carbonate. The percent of non-film-forming ingredients present in a coating, once dried, is generally known in the coatings industry as the pigment-volume-concentration (PVC). PVC is limited to ensure encapsulation of the non-film-forming ingredients by the latex polymer. If encapsulation becomes incomplete, the result is porosity in a dried coating that has diminished physical properties. In such a case, the dried coating is said to have exceeded the critical pigment-volume-concentration (CPVC). There is also a practical limitation in the amount of total solids that can be added to a water-borne coating since processing considerations such as manufacturing, application equipment and application rates are limited by the high viscosity and shelf life stability of the liquid coating.
High solids, water-borne coatings are desirable for many end-uses, and in particular roof coatings, where dried film thickness (DFT) requirements are higher than normal and application of multiple coats is necessary for reasonable dry rates and high coverage. Elastomeric properties, in roof coatings for example, are particularly important to withstand the stresses and strains associated with slow drying on a non-porous substrate such as a roof. Elastomeric properties are important to prevent excessive shrinkage and specifically, cracking and lifting of the coating away from the substrate as the water evaporates. The dried coating must have satisfactory elongation, usually at least 200% elongation to withstand substrate movement caused by the wide temperature range which is characteristic of roofing environments. A conventional water-borne, reflective top coat typically has about 50% solids (NV/Volume). Since top coatings are formulated to produce an "elastomeric" dried coating that has a minimum elongation of about 200%, the volume of non-film-forming ingredients in the dried coating is less than about 50% (less than about 50% PVC).
Conventional water-borne coatings are commonly applied as a reflective top coating for old and new roofing systems. For example, built-up and modified bitumen roofing systems typically require the application of a black asphalt base coat followed by at least two coats of reflective, and in some cases, fire resistant top coat. Suitable coverage by a roof coating system typically requires the application of a 50% NV/vol. asphalt base coat (applied at about 5.0 gallon per square and 40 dry mils thickness), followed by the application of two or more coats of 50% NV/vol. reflective, water-borne top coat (applied at approximately 1.0 to about 1.5 gallon per square and 8 to 12 dry mils thickness). The asphalt base coating requires a 30-day drying period before it can be top coated with the first coat of water-borne, reflective topcoat. A second coat of the water-borne topcoat is typically applied after 24 hours to complete the dried coating total thickness requirements of about 48-52 dry mils for the application. This lengthy application process is sometimes cause for further delay between coats because the incidence of ponded water can slow the drying of these multiple coat systems significantly.
There is a need to develop a water-borne surface coating that dries at a faster rate and yields an improved coverage rate. Also, there is a need to develop a surface coating that dries at a faster rate yet maintains or improves the elongation properties of conventional coatings. For practical purposes, the viscosity of the liquid coating must be within a range such that the coating can be applied with standard airless spray, brush or roller equipment.